1. Field of the Invention
The present invention relates generally to a multimode optical fiber and method for use with telecommunication systems employing low data rates, as well as systems employing high data rates, and more particularly, to a multimode optical fiber and method optimized for applications designed for state of the art laser sources, as well as common light emitting diode sources.
While the present invention is subject to a wide range of applications, it is particularly well suited for use in telecommunications systems designed to transmit data at rates equal to and exceeding one gigabit/sec.
2. Technical Background
The goal of the telecommunication industry is generally to transmit greater amounts of information, over longer distances, in shorter periods of time. Over time, it has been shown that this objective is a moving target with no apparent end in sight. As the number of systems users and frequency of system use increase, demand for system resources increases as well.
Until recently, data networks have typically been served by Local Area Networks (LANs) that employ relatively low date rates. For this reason, Light Emitting Diodes (LEDs) have and continue to be the most common light source in these applications. However, as data rates begin to increase beyond the modulation capability of LEDs, system protocols are migrating away from LEDs, and instead, to laser sources. This migration is evidenced by the recent shift toward systems capable of delivering information at rates equal to and exceeding one (1) gigabit/sec.
While such transmission rates will greatly enhance the capabilities of LANs, it does create an immediate concern for system owners. Multimode optical fiber currently employed in telecommunication systems is designed primarily for use with LED sources and is generally not optimized for use with the lasers envisioned to operate in systems designed to transmit information at rates equal to or greater than one (1) gigabit/sec. Laser sources place different demands on multimode fiber quality and design, compared to LED sources. Historically, the index profile at the core of multimode fibers has been tuned to produce high bandwidth with LED sources, which tend to overfill the core. The combination of the light intensity distribution from the LED source input pulse and the index profile of the fiber produces an overfilled modal weighting that results in an output pulse that has a relatively smooth rise and fall. Although peaks or plateaus resulting from small deviations from the ideal near-parabolic index profile do occur, their magnitude does not impact system performance at low data rates. In laser based systems, however, the intensity distribution of the source concentrates its power near the center of the multimode fiber. Consequently, small deviations in the fiber profile can produce significant perturbations in the impulse rise and fall, which can have a large effect on system performance. This effect can manifest itself in the form of excessively low bandwidth, as excessively high temporal jitter, or both. Although it is possible to correct these deficiencies to some degree by changing the launch condition of the source, such as the offset launch mode conditioning patch cord or the laser beam expander, this is typically not a practical solution for system owners.
A typical campus layout for a LAN system is designed to meet certain specified link lengths. The standard for the campus backbone (which travels between buildings) typically has a link length of up to about 2 km. The building backbone or riser (which travels between floors of a building)typically has a link length of up to about 500 meters. The horizontal link length (which travels between offices on a floor of a building) typically has a link length of up to about 100 meters. Older and current LAN technology, such as 10 Megabit Ethernet, can achieve a 2 km link length transmission with standard grade multimode optical fiber. However, next generation systems capable of gigabit/sec. and higher transmission rates cannot achieve all of these link lengths with standard multimode fiber presently available. In the 850 nm window, standard multimode fiber is limited to a link length of approximately 220 meters. In the 1300 nm window, standard grade fiber is limited to a link length of only about 550 meters. Accordingly, present technology only enables, at most, coverage for about two of the three campus link lengths. To fully enable a LAN for gigabit/sec. transmission rates, a multimode fiber capable of transmitting information over each of the three link lengths is necessary.
As used herein, overfilled (OFL) bandwidth is defined as the bandwidth using the standard measurement technique described in EIA/TIA 455-51 FOTP-51A, xe2x80x9cPulse Distortion Measurement of Multimode Glass Optical Fiber Information Transmission Capacityxe2x80x9d, with launch conditions defined by EIA/TIA 455-54A FOTP-54 xe2x80x9cMode Scrambler Requirements for Overfilled Launching Conditions to Multimode Fibersxe2x80x9d.
As used herein, laser bandwidth is defined as and measured using the standard measurement technique described in EIA/TIA 455-51A FOTP-51 and either of the following two launch conditions methods. Method (a) is used to determine the 3 dB bandwidth at 1300, and method (b) is used to determine the 3 dB bandwidth at 850 nm. Method (a), which is used to determine the 3 dB laser bandwidth at 1300 nm, utilizes a 4 nm RMS spectral width 1300 nm laser with a category 5 coupled power ratio launch modified by connection of a 2 meter, standard step index, single-mode fiber, patch-cord wrapped twice around a 50 mm diameter mandrel. The launch condition is further modified by mechanically offsetting the central axis of the singlemode fiber from that of the multimode fiber in such a manner that a 4 um lateral offset between the central axis of the core of the single mode fiber patch-cord and the multimode fiber under test is created. Note: category 5 coupled power ratio is described in and measured using procedures in TIA/EIA 526-14A OFSTP 14 appendix A xe2x80x9cOptical Power Loss Measurements of Installed Multimode Fiber Cable Plant. Method (b), which is used to determine the 3 dB laser bandwidth at 850 nm, utilizes a 0.85 nm RMS spectral width 850 nm OFL launch condition, as described in EIA/TIA 455-54A FOTP 54, connected to a 1 meter length of a specially designed multimode fiber having a 0.208 numerical aperture and a graded index profile with and alpha of 2. Such a fiber can be created by drawing down a standard 50 xcexcm diameter core multimode fiber having a 1.3 index of refraction delta (delta=no2xe2x88x92nc2/2nonc, where no=the index of refraction of the core and nc=the index of refraction of the cladding) to a 23.5 xcexcm diameter core.
Today, in order to increase distance, manufacturers typically shift bandwidth between two wavelength windows by changing the shape of the refractive index profile. Depending upon the changes made, the result is either high OFL bandwidth at the 850 nm window with low OFL bandwidth at the 1300 nm window, or low OFL bandwidth at the 850 nm with high OFL bandwidth at the 1300 nm window. For example, for a standard 2% Delta 62.5 um FDDI-type fiber, the refractive index profile can be adjusted to result in OFL bandwidth of 1000 MHz.km at 850 nm and 300 MHz.km at 1300 nm, or it can be adjusted to result in OFL bandwidth of 250 MHz.km at 850 nm and 4000 MHz.km at 1300 nm. With such multimode optical waveguide fibers having standard xe2x80x9calphaxe2x80x9d profiles, however, it is not possible to achieve an OFL bandwidth of 1000 MHz.km at 850 nm and 4000 MHz.km at 1300 nm. More typically, manufacturing tolerances would allow 850 nm/1300 nm OFL bandwidths of 600 MHz.km/300 MHz.km or 200 MHz.km/1000 MHz.km but not 600 MHz.km/1000 MHz.km.
There is a disconnect, however, between these historical bandwidth shifts, and what is necessary for gigabit/sec. transmission rates. Because high speed lasers are the standard light source for LANs designed to deliver information at rates exceeding a gigabit/sec., a multimode optical fiber having increased bandwidth at both the 850 nm and 1300 nm window is desired.
Moreover, because such LANs are in their infancy, all of the system components necessary to meet and/or exceed transmission rates of one gigabit/sec. are not yet fully reduced or practiced, optimized, and/or tested. For these reasons, it is not practical to replace existing LAN systems with a new LAN system speculatively designed to meet or exceed such high data rates. While it may be possible to achieve this result, it will likely not be the preferred or optimal solution, as following such a course of action will likely result in costly upgrades to the system and potentially a rework of the entire system.
The present invention is directed to a multimode optical fiber that is optimized for high speed laser sources capable of 1.0, 2.5, and 10 gigabit per second data transmission while exceeding the link length requirements discussed above. Moreover, the same multimode optical fiber maintains sufficiently high OFL bandwidth to support the transmission of information with the 1300 nm and 850 nm LED sources presently used in LAN systems. Such a multimode optical fiber will enable current LAN system owners to maintain their present LED based LAN systems, while at the same time enable them to easily transfer to a xe2x80x9cGigabit Ethernet Systemxe2x80x9d without having to undertake a costly multimode fiber upgrade. As used herein, xe2x80x9cGigabit Ethernet Systemxe2x80x9d is defined as a telecommunication system, such as a LAN, which is capable of transmitting data at rates equal to and/or exceeding one (1) gigabit/sec.
Accordingly, one aspect of the present invention relates a multimode fiber having a first laser bandwidth greater than 220 MHz.km in the 850 nm window, a second laser bandwidth greater than 500 MHz,km in the 1300 nm window, a first OFL bandwidth of at least 160 MHz.km in the 850 nm window, and a second OFL bandwidth of at least 500 MHz.km in the 1300 nm window. Such a multimode optical fiber has a variety of uses in the telecommunication industry, and is particularly well suited for use in telecommunication systems employing high speed laser sources. Such a fiber has the added benefit of providing sufficient OFL bandwidth for LED sources presently used in LAN systems.
In another aspect, the invention is directed to a multimode transmission system capable of transmitting data at rates equal to and exceeding one gigabit/sec. The multimode transmission system includes a laser source which transmits at least one gigabit/second of information, and a multimode optical fiber communicating with the laser source. The multimode optical fiber has a first laser bandwidth of at least 385 MHz.km in the 850 nm window which is capable of carrying the information at least 500 meters. The multimode optical fiber also has a second laser bandwidth of at least 746 MHz.km in the 1300 nm window for carrying the information at least 1000 meters. In addition, the multimode optical fiber includes first and second OFL bandwidths sufficiently high to be used with 850 nm and 1300 nm LED sources.
Another aspect of the present invention relates to a multimode optical fiber having a 62.5 xcexcm core, and a cladding bounding the core. The cladding has a refractive index lower than the refractive index of the core, and the multimode optical fiber exhibits a DMD profile, which when measured at a wavelength of 1300 nm, includes a first region having an average slope measured from (r/a)2=0.0 to 0.25, and a second slope region having an average slope measured from (r/a)2=0.25 to 0.50. The slope of the first region is preferably greater than the slope of the second region. More preferably, the slope of the first region is greater than 1.5 times the slope of the second region.
In a further aspect, the present invention is directed to a method of forming a multimode optical fiber. The method includes the steps of thermochemically reacting a silica containing precursor reactant and at least one dopant reactant to form soot, and delivering the soot to a target in a manner sufficient to produce a glass preform having specified characteristics. The glass preform is drawn into a multimode optical fiber having a 62.5 xcexcm core region and a cladding region bounding the core region. The reacting step includes selecting a precursor reactant and a dopant reactant according to a soot deposition recipe sufficient to result in a multimode optical fiber which exhibits a DMD profile, which when measured at a wavelength of 1300 nm, has a first average slope measured over a first region from (r/a)2=0.0 to 0.25, and a second average slope measured over a second region from (r/a)20.25 to 0.50, the first average slope being greater than the second average slope.
The multimode optical fiber of the present invention results in a number of advantages over other multimode optical fibers known in the art. One such advantage is that the multimode optical fiber of the present invention is fully compatible for use with high speed laser sources, as well as LED sources. Accordingly, the multimode optical fiber of the present invention can be used with conventional local area networks employing LED sources, and can be used with Gigabit Ethernet Systems, which employ high speed laser sources.
In addition, the multimode optical fiber of the present invention eliminates the need for costly mode conditioning patch cords often used to enable operation in the 1300 nm operating window for Gigabit Ethernet System protocol. For many multimode optical fibers, a mode conditioning patch cord is used to move power away from the center of the multimode fiber in order to avoid center line profile defects which typically result from some manufacturing processes. Because the preferred multimode optical fiber of the present invention is manufactured using the Outside Vapor Deposition process (OVD), the preferred multimode optical fiber of the present invention has reduced centerline profile defects. Accordingly, a mode conditioning patch cord is no longer needed to enable operation in the 1300 nm operating window of the preferred fiber of the present invention, thus allowing for on-center launch or slightly off-set due to loose connector tolerances, resulting in ease of installation and use.
Moreover, the multimode optical fiber of the present invention optimizes laser performance with a variety of laser sources, such as, but not limited to, 780 nm Fabry-Perot lasers, 850 nm Vertical Cavity Surface Emitting Lasers (VCSELs), 1300 nm Fabry-Perot lasers, and low cost 1300 nm transmitters envisioned for the future. The multimode optical fiber of the present invention is also designed to support operation at 2.5 and 10 gigabits/second over significant link lengths when used with high performance lasers in more advanced telecommunication systems.
Additional features and advantages of the invention will be set forth in the detailed description which follows, and in part will be readily apparent to those skilled in the art from that description or recognized by practicing the invention as described herein, including the detailed description which follows, the claims, as well as the appended drawings.
It is to be understood that both the foregoing general description and the following detailed description are merely exemplary of the invention, and are intended to provide an overview or framework for understanding the nature and character of the invention as it is claimed. The accompanying drawings are included to provide a further understanding of the invention, and are incorporated in and constitute a part of this specification. The drawings illustrate various embodiments of the invention, and together with the description serve to explain the principles and operation of the invention.